1. Field of the Invention
This invention relates to an improvement of the ion-extracting grid of an ion beam machining device, and more particularly to an ion-extracting grid which produces a high ion beam current density with a low accelerator voltage.
2. Description of the Prior Art
To produce the fine pattern of integrated circuits, the reactive etching process has been widely used. However, the demand for higher degree of integrity in the integrated circuits is ever increasing, and the capability of the reactive etching process in producing fine patterns has reached a limit. An ion beam machining process has been developed for producing patterns or forms which are finer than that producible by the reactive etching process. The ion beam machining process extracts ions of beam form with a high energy (more than 10 eV) from an ion source and uses the ions of beam form for shaping fine patterns, shaping thin films, transforming surface, and the like. Such ion beam machining process has been used fairly frequently for the above-mentioned shaping and transforming. Since its operating principle is in the mutual actions among particles, the ion beam machining process can shape patterns in the order of several tens of angstrom (several 10 .ANG.).
Various ion sources are available for the ion beam machining process. An example is the Kaufman type ion source capable of producing ion beams with energy in a range of 10 eV to more than 500 eV, which ion beams have been used for sputtering to form thin films and fine patterns. The Kaufman type ion source has advantages in that ion beams with an energy level suitable for sputtering can be produced by using a simple control means and that its workability and its reproducibility are high, but it has shortcomings in that a high beam current density cannot be achieved with a low accelerator voltage and that it requires a long working or processing time. In the Kaufman type ion source, if the accelerator voltage is raised with an intention of producing a high beam current, the density of the ion beam current increases but the energy of the ion beams becomes too high and various difficulties such as damage to a mask are caused. Accordingly, there has been a pressing need for the development of an ion source which not only produces ion beams with energy in a range suitable for sputtering but also generates high beam current densities by using a low accelerator voltage.
FIG. 1 shows a schematic sectional view of a conventional ion beam machining device having a Kaufman type ion source. In general, the ion beam machining device of this type has a plasma-generating chamber 1 for generating plasma ions, an ion-extracting grid 2 for extracting the generated ions in the form of ion beams, and a processing chamber 3 for irradiating the ion beams to a workpiece.
The plasma-generating chamber 1 includes a cathode 4 disposed at the central portion of a cylindrical sealing wall 5 which seals the chamber 1. A cylindrical anode 6 is disposed inside of the cylindrical sealing wall 5. An electromagnet 7 surrounds the outside of the sealing wall 5 in such a manner that vertical magnetic field is produced in the plasma-generating chamber 1, so as to produce cyclotron motion of electrons emitted from the cathode 4. To generate glow discharge between the cathode 4 and the anode 6, an anode power source 8 of 40-50 V is connected thereto so as to keep the anode 6 positive relative to the cathode 4.
The ion-extracting grid 2 has a screen grid 9 for defining a plasma boundary and an accelerator grid 10. The screen grid 9 is provided with a large number of holes and kept at the same potential as that of the sealing wall 5. The accelerator grid 10 has a large number of holes for accelerating ions emanating from the plasma boundary by thermal motion thereof. The screen grid 9 and the accelerator grid 10 are spaced by a predetermined distance while the holes of the two grids are aligned each other. An accelerator power source 11 is connected so as to keep the accelerator grid 10 negative relative to the screen grid 9. To prevent reverse movement of electrons from the processing chamber 3, the accelerator grid 10 is kept negative to the workpiece or target by a suppressor power source 12. A target holder 13 is disposed in the processing chamber 3, so as to hold a workpiece 14 thereon, and a mask 15 is placed on the workpiece 14.
The generation of plasma and irradiation of the ion beams on the workpiece 14 will be described now. The ion beam processing device is evacuated by an evacuating means (not shown) through an opening 16 to 10.sup.-6 to 10.sup.-7 Torr, and argon (Ar) gas is fed therein through another opening 17. As being emitted from the cathode 4 by the thermoelectronic emission, the electrons are caused to make cyclotron motion by the magnetic field of the electromagnet 7 until they collide the anode 6. In the course of motion, the electrons are accelerated by the electric field between the cathode 4 and the anode 6, and their collision with argon atoms ionizes the latter so as to produce Ar.sup.+ ions, whereby glow discharge is caused and a low-ionization plasma is produced.
Thin ion sheaths are formed between the thus produced plasma and the sealing wall 5 and between the plasma and the screen grid 9, and plasma boundaries are defined by such ion sheaths. Ions emitted from the plasma boundaries by thermal motion are accelerated by a gross accelerator voltage corresponding to the difference between the plasma potential and the potential of the accelerator gird 10 (more specifically, a value obtained by subtracting the suppressor power source voltage from the difference between the plasma potential and the potential of the accelerator grid 10), so that the thus accelerated ions are extracted in the form of ion beams. The ion beams are irradiated onto the workpiece 14 as an ion shower in the processing chamber 3. Accordingly, those portions of the workpiece 14 which are not covered by the mask 15 are directly bombarded by the ion beams, and the workpiece 14 itself is etched to produce a fine pattern thereon after the shape of the mask 15.
The maximum current density J (A/cm.sup.2) available in the Kaufman type ion beam machining device is given by the following equation. EQU J=n(.pi..epsilon..sub.o /9)(2q/Mi).sup.1/2 (ds/le).sup.2 V.sub.T.sup.3/2( 1)
here,
n: density of holes in the grids, PA1 q: electric charge of the ion, PA1 Mi: mass of the ion, PA1 ds: diameter of the hole of the grid, PA1 V.sub.T : Maximum accelerator voltage, PA1 le: (lg+ds.sup.2 /4).sup.1/2, PA1 lg: distance between the screen grid and the accelerator grid.
As can be seen from the equation (1), a first method to increase the maximum current density J is to increase the grid hole density n (number of grid holes per unit area). There is a certain limit as to the material of the grid, because the grid is required to have a certain mechanical strength, a certain heat resistivity, and a certain durability against sputtering due to ion beams being extracted. To meet the above requirements of the physical properties, carbon grids have been used, and from the standpoint of machining there is a certain limit in increasing the number of holes per unit area of the carbon grid, or the grid hole density. Especially, when the two grids, i.e., the screen grid 9 and accelerator grid 10, are overlaid one over the other while aligning the holes thereof, it is extremely difficult to make the grid holes small.
As a second method for increasing the maximum current density J, one may consider the decrease of the distance lg between the screen grid 9 and the accelerator grid 10. However, a certain minimum spacing is indispensable between the two grids for preventing breakdown of the insulation therebetween.
As a third method for increasing the maximum current density J, the use of a high accelerator voltage may be considered. However, this method tends to cause an excessively high accelerating energy of the ion beams which causes mask contamination by sputtering or the like difficult.
Thus, the ion-extracting grid in the conventional Kaufman type ion source has shortcomings in that high beam current densities cannot be obtained and that a long processing time is necessary for sputtering. Besides, the construction, which uses the two perforated grids to be assembled while aligning the holes thereof, requires high accuracy both in the machining and in mounting onto the ion beam machining device, and the material for the ion-extracting grid is limited.
As a means to solve the above-mentioned shortcomings, it has been proposed to eliminate the screen grid so as to form the ion-extracting grid by the accelerator grid alone. However, the elimination of the screen grid results in a number of shortcomings, such as an increased angle of beam divergence, exposure of the grid itself to machining by the ions resulting in a shortened service life of the grid, contamination of the workpiece, and the like.